The present invention relates to a copier or similar image forming apparatus and, more particularly, to a finisher connetable to an image forming apparatus for sorting and, if necessary, binding sheets driven out of the apparatus and distributed to a plurality of bins thereof which are arranged one above another.
A finisher for the above application is often implemented as a sheet distribution type finisher which receives a desired number of sets of copy sheets of the same document sheets from an image forming apparatus and sequentially distributes the copy sheets to the same number of bin trays thereof as the number of sets of copy sheets. It is a common practice with this type of finisher to fix the bin trays in place and shift a discharge position for discharging sheets to the bin trays or to fix the discharge position and sequentially move the bin trays relative to the discharge position. For example, Japanese Patent Laid-Open Publication No. 4855/1982 discloses a sorting device which increases the clearance between the bin tray to receive a sheet and the immediately overlying bin tray by use of a Geneva wheel so as to facilitate the receipt of a sheet. Japanese Patent Laid-Open Publication 78769/1981 proposes an improved sorting device which increases the above-mentioned clearance by use of a helical cam. Further, Japanese Patent Laid-Open Publication No. 110075/1990 teaches a sorting device having bin trays movable up and down and a cam for causing one of the bin trays for receiving a sheet to move in a direction parallel to the stacking surface thereof. This is successful in forming a sheet inlet without increasing the clearance between nearby bin trays.
The conventional finishers each has a discharge roller located at a discharge position adjacent to the bin trays for transporting sheets driven out of an image forming apparatus to the bin trays. Specifically, the discharge roller is spaced apart by a predetermined distance from a trailing edge positioning fence (simply referred to as a fence hereinafter) which is provided on each bin, requiring a sheet to fly to the bin tray over the fence of the bin tray. To meet this requirement, the discharge roller is rotated at a high speed to discharge a sheet at an initial speed high enough for the sheet to fly the predetermined distance to the bin tray. This brings about a problem that when the distance which a sheet flies is short, the rear edge of the sheet fails to go over the fence of the bin tray while, if the distance is excessive, the sheet flies beyond a sheet stack existing on the bin tray and cannot be neatly arranged together with the sheet stack. Especially, when the sheet is soft or warped, it is likely that the leading edge of the sheet being driven out by the discharge roller toward the bin tray is inclined in the right-and-left direction or is offset in the same direction when dropped onto the bin tray. Another problem is that a sheet dropped onto the bin tray slides down on the existing sheet stack while flying in the right-and-left direction due to the friction between the sheet and the sheet stack, static electricity, air stream, etc. Such a sheet abuts against the fence of the bin tray, further springs up, an then drop to a different position. In this manner, the sheet brought to a stop in abutment against the fence is noticeably irregular in the right-and-left position. This type of finisher, therefore, needs a positioning device for correcting the position of sheet on the bin tray. A positioning device is also essential to to finisher having a binding function or a punching function in order to provide a finished sheet stack with attractive appearance.
In an intended direction of sheet transport, the finisher usually positions a sheet on the bin tray by causing the sheet sliding down along the sacking surface of the bin tray due to gravity to abut against the fence. In a direction perpendicular to the direction of sheet transport, the finisher positions the sheet by a sheet positioning device having a pressing member which is movable from a standby position in the above direction for urging the sheet against a reference wall by pressing one side thereof. Sheets of various sizes are distributed to the bin trays of the finisher, as needed. A control device is incorporated in the finisher for causing the sheet positioning device to perform a positioning action matching the sheet size. Specifically, the controller controls the distance which the pressing member moves from the standby position in response to size information sent from a sensor located on a transport path or sent from the image forming apparatus. As a result, the pressing member slightly presses one side of the sheet, retracts to and stops at the position corresponding to the side of the sheet, and then fully returns to the standby position. The pressing member may be provided on each of the bin trays so as to slide on the stacking surface of the bin tray. Alternatively, the pressing member may be constituted by a single elongate member extending through all the bin trays for positioning sheets on the bin trays at the same time.
As stated above, to position a sheet on the bin tray, the pressing member of the positioning device is driven by a motor to move a distance matching the sheet size, thereby pressing one side of the sheet. When the sheet has a size smaller than the minimum size which can be dealt with by the positioning device, the binding function or the punching function is inhibited and only the sorting function is effected. However, when the size of the sheet is far smaller than the minimum allowable size, the sheet brought to a stop in abutment against the fence is apt to drop to either side of the existing sheet stack and practically fail even to overlap the sheet stack. Then, it is likely that the order of the sheet stacks are different from expected one when they are taken out of the bin tray.